Let's clarify
For what i know, soundstage is the brain/human perception of imaging.
Then, again based on my very little experience, imaging should be the capability of the earphones to place instruments and voices where they should be, like if they were around you or in front of you and so on..
The soundstage is the impression you have when the imaging is good, so you can perceive like if you're in a room when you are listening to instruments thar sound around you.
So the difference is that imaging is the technical capability of positioning and placing the instruments in the space when they're supposed or intended to be by the technician or musicist, the soundstage is the perception that you have when the earphone got a good imaging.
I can be wrong so if someone knows it better, let' explain.
Many times anyway the two words are used with the same meaning as far as i know.
In any case, i advise you to take a look to the Head-fi glossary which collects many words from this world and can help you distinguish the various words that sound different but maybe mean the same thing.